The invention proceeds from a releasable check valve which is intended to be useable for very high system pressures.
Check valves readily permit pressure medium to flow from a first orifice to a second orifice with the closing element being raised from a seat counter to the force exerted by the pressure prevailing in the second orifice and counter to the force of the closing spring by means of a force produced by the pressure in the first orifice. Since the closing element is conventionally acted upon by the pressures on surfaces of identical size, a pressure arises in the first orifice which is higher than the pressure in the second orifice by a pressure difference which is equivalent to the force of the closing spring. The closing spring is only weak so as to keep the losses via the valve small unless the intention is to deliberately build up the pressure medium in the first orifice. The flow through a check valve in the direction from the second orifice to the first orifice is possible only by additional measures through which a releasable check valve is provided. A poppet part is then provided which can act on the closing element in the opening direction and can raise it from the seat counter to the force of the closing spring and counter to the pressure difference between the first and second orifice.
DE 197 14 505 Al has disclosed a releasable check valve. The internal high-pressure forming of tubular semi-finished products is mentioned in the abovementioned document as an example of use of a check valve of this type. The check valve which is shown has a valve housing having a continuous receiving bore which is stepped and is composed essentially of three sections. The diameter of the receiving bore is larger in the two outer sections than in a central section into which the two outer sections merge in steps lying in radial planes. High-strength inserts which are exposed to the system pressure and inserts serving to guide a poppet piston are placed into the two outer sections. The central section of the receiving bore is divided by a piston collar of the poppet piston into two annular spaces of which the one can be acted upon by control pressure via a pilot valve in order to control the check valve or can be relieved from pressure to a tank, and the other is permanently connected to a pressure-medium reservoir and contains a restoring spring for the poppet piston.
It has been found that in the known check valve the poppet piston is not always smooth-running to the desired extent. In addition, the sealing action and the service life of various sealing arrangements between the poppet piston and the inserts were not always unproblematical.
The invention is therefore based on the object of developing a releasable check valve in such a manner that the poppet piston is not stiff beyond the extent to be expected because of the frictional forces at the seals and a high sealing action and service life of the seals is possible.
According to the invention, this object is achieved in a releasable check valve. The invention is first of all based on the finding that stiffness of the poppet piston, deficient action and limited service life of the seals in the known valve are caused by alignment errors. These in turn are attributed to the fact that those sections of the receiving bore which receive the inserts are machined from different sides of the valve housing. According to the invention, the receiving bore now has essentially the same diameter in the region of the inserts and between the latter, apart from short turned grooves in the axial direction which may be present, so that said receiving bore can be machined from just one side of the valve housing, i.e. without changing the position of the valve housing or of the tool. Arranged axially between two inserts is a spacer bushing which ensures a fixed spacing between two inserts on different sides of the piston collar even when the diameter of the receiving bore is constant. At the same time, by means of the spacer bushing and the piston collar, the two annular spaces on the two sides of the piston collar are separated fluidically from each other, preferably with an additional seal being used.
It is preferred for the two inserts bearing directly against the two end sides of the spacer bushing to be identical to each other, so that the number of different components is small.
In a releasable check valve according to the invention, two inserts may be situated on one side of the spacer bushing, which is advantageous, for example the installation of seals. If a releasable check valve according to the invention is used at very high system pressures, which may amount to 4000 or 6000 bar in internal high-pressure forming, for example, then it is important that the separating gaps between the two inserts are relieved from pressure so that the high system pressure cannot build up there. This is because this pressure would produce such a force at the mutually facing end sides of the radially relatively large inserts that there would be the risk of the valve ripping apart. An annular channel also lies between the inserts and the valve housing in the fluid path for the pressure relief. It is then important for the separating gap between the two inserts to be situated in the region of the annular channel. A check is then possible in a simple manner by the fact that the outermost of the inserts on the one side of the spacer bushing has an edge or surface which, in the designated position of the inserts with respect to the annular channel, lies flush with an end surface of the valve housing.
The effect achieved by the refinement is that the annular spaces on the two sides of the piston collar cannot be acted upon by maximum pressure from the two end sides of the end sections of the poppet piston. To this end, two axially spaced-apart seals are provided between which the gap between the end sections of the poppet piston and an insert is relieved from pressure via a leakage oil connection. The particular refinement of this pressure relief ensures that part of the sealing arrangement in the open turn-out of the second insert can bear axially against a smooth surface of the first insert, which surface does not have any discontinuities.